IDAX and MIDAX printing techniques are commercial electrographic imaging techniques that utilize what is referred to as silent electric discharge. In such systems, an ion cartridge is mounted adjacent an imaging drum. The drum then moves into contact with a transfer sheet (e.g. paper). The conventional cartridges utilized in these printing systems include first and second electrodes, typically called the driver and control electrodes, separated by a solid dielectric member, such as a sheet of mica. The control electrode, typically in the form of control fingers, defines an edge surface disposed opposite the driver electrode to define a discharge region at the junction of the edge surface and the solid dielectric member. An alternating potential is applied between the driver and control electrodes of sufficient magnitude to induce charged particle producing electrical discharges in the discharge region, and means are provided for applying a charged particle extraction potential between the control electrode and a further electrode, so that imaging occurs on the imaging drum, or paper or like dielectric moving past the ion cartridge. In most commercial installations a screen electrode is also provided, between the imaging drum and the control electrode, and separated by an insulating spacer from the control electrode. A commercial ion cartridge is typically constructed of a plurality of driver, control, and screen electrode units, in a matrix form.
In commercial installations of MIDAX printers, there typically are three major manners in which the ion cartridges fail. The spot size produced by the ion cartridge grows as the cartridge ages, and once it gets to a particular level so that the print quality is unacceptably poor, the cartridge must be cleaned or retired; or under some circumstances there is catastrophic failure of the cartridge.
One conventional way in which ion cartridges fail is euphemistically referred to as "red death". By-products formed in the ionization process, such as oxides, build up on the cartridge control fingers which can cause an uneven rush of electrons and negative ions upon application of the extraction voltage. Another mode of failure is euphemistically referred to as "white death". In the white death scenario, white crystals, which typically are nitrates, build up on the screen electrode thereby creating a dielectric layer and causing an electrical defocussing of the electron and ion stream as it exits the cartridge. A third typical mode of failure, euphemistically referred to as "black death", is caused by premature catastrophic failure of the cartridge when conductive toner is sucked up into the cartridge and creates unwanted electrically conductive paths and also localized heating.
According to the invention it has been found that the mechanisms by which at least red and white death occur are dependent upon the characteristics of the atmosphere from which the ions are produced by the ion cartridge. The atmosphere is typically normal air, although it may be contaminated with ammonia, benzene, or other gases depending upon the particular plant in which the system is utilized. Nitrogen, oxygen, and water vapor are the major components of the atmosphere, and during operation of the MIDAX printers after one stream of electrons and ions is created and extracted from the cartridge new air replaces that which was lost from the cartridge. Most of the problems of ion cartridge aging are caused by compounds made of or initiated by oxygen and/or water vapor, and therefore the process can be slowed or even eliminated by the replacement of the air around the ion cartridge with appropriate other gases. Even in situations where ion cartridge life is not extended, however, there may be significant advantages to providing a particular atmosphere in the ion cartridges. For example the quality of the print--its uniformity--may be significantly enhanced. Uniformity enhancements on the order of 40% are not unusual when the atmosphere from which the electrons and ions are created by the ion cartridge is properly controlled.
According to the present invention, it has been found that if a substantial portion of the air at the discharge region of the ion cartridge is replaced with nitrogen, elemental noble gases, mixtures of noble gases, or mixtures of nitrogen with one or more noble gases, uniformity and/or cartridge life can be significantly enhanced. If the gas is supplied in a particular manner even black death catastrophic failure can be eliminated or minimized.
Gases that are particularly effective in the practice of the invention are nitrogen, mixtures of nitrogen and helium, and mixtures of nitrogen with argon, xenon, neon, and/or krypton. It has been found that completely dry pure nitrogen is not particularly effective since nitrogen is not easily ionized, and therefore there must be some "catalyst" present to enhance the nitrogen ionization. However the catalyst must be present in small enough amounts so that arcing does not occur, since arcing can be destructive and reduce cartridge life. While water vapor that naturally occurs can provide this catalyst effect, it is desirable for other reasons to keep the amount of water vapor to a minimum. Therefore it is most desirable to add another gas, such as a noble gas, to the nitrogen.
While helium can be effective as a catalyst for nitrogen ionization, if helium is used in a commercial environment it can be dangerous to a human operator since the helium and nitrogen ionization may generate gases that would make an operator dizzy. Argon, xenon, neon, and krypton do not have that effect, however, yet they provide an effective catalyst for nitrogen ionization. The amounts of argon, neon, krypton, or xenon must be controlled, however, to make sure that they are low enough so that arcing does not occur.
In the preferred form of the present invention, nitrogen is mixed with argon, xenon, neon, or krypton so that there is a volume ratio of about 5 - 1 to about 20 - 1 of nitrogen to other gas. The invention is most effective in some actual operating environments when nitrogen and argon are mixed at a ratio of about 10 to 1. Typically the gas mixture is supplied to the discharge region at a rate of about 4.75-6.25 cubic feet per hour, typically about 0.5 cubic feet per hour of argon, xenon, neon, or krypton, and about 5 cubic feet per hour nitrogen.
A number of particularly advantageous mechanisms for introducing the gas to the discharge region are provided according to the invention. Black death can be significantly reduced if the gas is introduced through the insulating spacer between the control electrode and the screen electrode. The gas is typically introduced at a pressure above atmospheric pressure so that a positive pressure is provided in this area, and conductive toner can therefore not be easily sucked into the ion cartridge. Alternatively, the gas may be injected through a plenum and holes spaced about one-half inch along a pre-existing cartridge mounting rail, typically the first rail in the direction of rotation of the imaging drum. Alternatively, a pair of gas manifolds may be provided at opposing ends of the imaging drum, and a pair of spray tubes extending between the gas manifolds with a plurality of openings provided along their length. The gas is then supplied by regulators and conduits to the gas manifolds, and thus introduced uniformly between the ion cartridge and the imaging drum.
According to one aspect of the invention, there is provided apparatus for generating charged particles for electrostatic imaging. The apparatus comprises: an imaging drum; an ion cartridge; a support between the image drum and the ion cartridge for supporting the ion cartridge; said ion cartridge comprising a solid dielectric member, a first electrode substantially in contact with one side of the solid dielectric member, a second electrode substantially in contact with the opposite side of the solid dielectric member with an edge of the second electrode disposed opposite the first electrode to define a discharge region at the junction of the edge surface and the solid dielectric member; means for applying potential between the first and second electrodes of sufficient magnitude to induce charged particle producing electrical discharges in the discharge region between the dielectric member and the edge surface of the second electrode and means for applying a charged particle extraction potential between the second electrode and a further electrode; a pair of gas manifolds at opposite ends of the drum; a pair of spray tubes, having numerous perforations along their length, extending between the gas manifolds, for supplying gas to the volume between the drum and the ion cartridge; and means for supplying a mixture consisting essentially of nitrogen gas, with an amount of argon, xeon, neon, or krypton effective to provide a catalyst for nitrogen ionization while preventing arcing, from the screen to the imaging drum.
According to another aspect of the invention, there is provided a method of generating charged particles for electrostatic imaging comprising the steps of: (a) applying an alternating potential between a first electrode substantially in contact with one side of a solid dielectric member and a second electrode substantially in contact with an opposite side of the solid dielectric member, said second electrode having an edge surface disposed opposite said first electrode to define a discharge region at the junction of the edge surface and the solid dielectric member, to induce charged particle producing electrical discharges in said air region between said solid dielectric member and the edge surface of said electrode; (b) applying a charged particle extraction potential between said second electrode and a further electrode member to extract charged particles produced by the electrical discharges in said air region; (c) applying the external charged particles to a further member to form an electrostatic image; and (d) supplying a gas to the discharge site to replace the vast majority of the air during charged particle generation, said gas being a mixture consisting essentially of the nitrogen, with an amount of argon, xeon, neon, or krypton effective to provide a catalyst for nitrogen ionization while preventing arcing. Step (d) is preferably practiced by providing the nitrogen and argon, xeon, neon, or krypton at a ratio of about 5 to 1 to about 20 to 1, e.g. by providing nitrogen and argon at a ratio of about 10 to 1.
It is the primary object of the present invention to provide for enhanced uniformity and/or enhanced cartridge life in silent electric discharge electrographic imaging. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.